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Nonlinear Magneto-Optical Studies in Magnetic Superlattices

This conference focuses on the nonlinear magneto-optical effect in magnetic superlattices, discussing topics such as the nonlinear Kerr rotation and surface sensitivity. Experimental setups and results will be presented.

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Nonlinear Magneto-Optical Studies in Magnetic Superlattices

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  1. 1st International Conference on Quantum Photonic Science Nonlinear Magneto-Optical Studies in Magnetic Superlattices and Magnetic Nano Structures K. Sato, A. Kodama, M. Miyamoto, M. Tsuruga, T. Matsumoto, T. Ishibashi Y. Morishita, Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan K. Takanashi, S. Mitani,Institute of Materials Science, Tohoku University, Sendai, Miyagi, Japan TUAT COE Project “Future Nano Materials”

  2. Nonlinear Magneto-optics • What is the nonlinear magneto-optical effect? Magnetization-induced nonlinear optics • What is the nonlinear Kerr rotation? When P-polarized primary light is incident both P- and S-polarized SH light emits: which leads to rotation of E vector from the plane of incidence. • In centrosymmetric materials such as Fe and Au no SHG occurs due to cancellation of P and –P.

  3. Azimuthal angle dependence of SHG from Si and GaAs wafer Si wafer (001) centrosymmetric GaAs wafer (001) Non-centrosymmetric

  4. Theoretical prediction and experimental verification • Nonlinear magneto-optical Kerr rotation larger than linear rotaion was theoretically predicted1), and was experimentally proved2,3). 1) W. Hübner and K.-H. Bennemann: Phys. Rev. B40, 5973 (1989) 2) Th. Rasing et al.: J. Appl. Phys. 79, 6181 (1996) 3) Th. Rasing: J. Mag. Soc. Japan 20 (Suppl. S1), 13 (1996)

  5. Surface and interface sensitivity of MSHG • Application of MSHG Sensitive to Evaluation of the break of Multilayers symmetry at surface Imaging of domains ・This effect cannot be expected to be applied to some practical memory devices but is thought to be useful for characterization of surfaces and interfaces of materials.

  6. Nonlinear magneto-optical effect linear response ・ For weak incident laser field E(w) : Nonlinear response ・ For strong incident laser field E(w) : Third rank tensor is not allowed in centrosymmetric materials. ・ Nonlinear polarization P(2) for incident field of E=E0sinwt Second harmonic generation (SHG)

  7. Nonlinear polarization of 2nd order parametric process SHG process Light rectification

  8. Definition of nonlinear susceptibility Centrosymmetric materials:all the χijk(2) components vanish. (from symmetry operations) Surfaces and interfaces: symmetry breaks, leading to appreciable amount of nonlinear magneto-optical effect even in the centrosymmetric materials

  9. Wave equation of linear magneto-optical effect YK=fK+ihK（複素カー回転角） c1(1)=eyz, c0(1)=exx-1=N2-1

  10. Wave equation of nonlinear magneto-optics. Source term does not depend on optical constants of materials, leading to special solution associated with the second order susceptibility.

  11. Nonlinear Kerr roation

  12. Nonlinear Kerr rotaion • Different from the linear case • χ(2)odd/χ(2)even contributes。 • This term is zero in centrosymmetric materials • And takes a finite value at surfaces • Surface sensitivityuseful for surface • magnetism studies！

  13. Difference between linear and nonlinear Kerr rotation Linear： factor reduces the magnitude Also χxy is order of magnitude smaller that χxx Nonlinear：no such factor exists Also χodd and χeven are of the same order 。

  14. Microscopic origin of MSHG 3 photon proicess SHG Here |k q//l >→|k+q//l‘>|k+q//l’>→|k+2q//l" > |kl> →|k+2q//l”> w w 2w Intermediate state Ground state Excited state

  15. Illustration of microscopic process of MSHG

  16. Nonlinear Kerr rotation of Fe

  17. Nonlinear Kerr rotation of Fe/Cu Nonlinaer Kerr effect nolinear

  18. Cu cover layer-thickness dependence of Co/Cu SH信号 Cuの層厚

  19. { N { 3 { 2 Au(xML) { 1 Fe(xML) Au buffer Fe seed MgO(100) 2 Au 1 Fe 0 0.25 0.5 0.75 1 x=3.75ML Superlattices ：〔Fe(xML)/Au(xML)〕N Integer : x=1, 2, 3, 4, 5, 6, 8, 10, 15 Non-integer : x=1.25, 1.5, 1.75, 2.25, 2.5, 2.75, 3.25, 3.5, 3.75

  20. N Au(xML) 2 Fe(xML) Au buffer layer 1 Fe seed layer MgO (100) [Fe(1ML)/Au(1ML)] Fe(1ML)/Au(1ML) superlattice 4.054Å bcc-Fe (001) Au Fe 2.867Å Schematic structure for the Fe/Au superlattice L10 fcc-Au (001) 4.079Å Atomic arrangement in a unit cell of Fe-Au with a L10 suructure.

  21. l=810nm Pulse=150fs P=600mW rep80MHz LD pump SHG laser Ti: sapphire laser Mirror l=532nm Electromagnet Filter Berek compensator Stagecontroller Mirror Sample Chopper Analyzer lens polarizer Lens Photon counting Filter PMT Photon counter Computer MSHG Measureing System

  22. Laboratory • Experimental setup for MSHG measurement

  23. Sample Optical Setups(Longitudinal Kerr) 試料回転 Sample stage w (810nm) P-polarized or S-polarized light Pole piece 45° Rotating analyzer w (810nm) Analyzer Filter 2w (405nm)

  24. Electromagnet 104 S-polarized light ω(810nm) 2K(2)=34.3 Rotating Analyzer 45° Analyzer SHG intensity (counts/10sec.) Filter 2w (405nm) Analyzer angle (deg.) Nonlinear Kerr Rotation and Kerr Ellipticity Result The curves show a shift for two opposite directions of magnetic field Analyzer angle-dependence for [Fe(3.5ML)/Au(3.5ML)] (Sin) Nonlinear Kerr rotation & ellipticity K(2)= 17.2  hK(2)=３°

  25. SHG intensity (counts/10sec.) Analyzer angle (deg.) Largest nonlinear Kerr rotationobserved in the Fe/Au series Df = 31.1° Fe(1.75ML)/Au(1.75ML)Sin

  26. Electromagnet Rotation of sample P-polarized light w (810nm) Analyzer 45° Filter 2w (405nm) Azimuthal Angle Dependence ・　Linear optical response(=810nm) The isotropic response for the azimuthal angle ・　Nonlinear optical response (=405nm) The 4-fold symmetry pattern Azimuthal pattern show 45-rotation by reversing the magnetic field nonlinear linear 45 SHG intensity (counts/10sec.) SHG intensity (counts/10sec.) (a)Linear (810nm) (b)SHG (405nm) Azimthal angle-dependence of MSHG intensity for [Fe(3.75ML)/Au(3.75ML)] superlattice. 　（Pin Pout）

  27. Pin-Pout Azimuthal Angle Dependence Sin-Sout

  28. Discussion The equation of the azimuthal angle-dependence by the theoretical analysis A: Surface nonmagnetic term 　・ The electric dipole origin give rise to isotropic signal. B: Bulk nonmagnetic term 　 ・ The quadrupole origin causes an anisotropic contribution for four rank tensor. C: Surface magnetic term 　・ The time reversal symmetry is lifted by magnetization 　・ Mirror symmetry operations should be supplemented with an additional reversion of M.

  29. Calculated azimuthal angle dependence of SHG and MSHG signals Kerr rotation calculated from parameters Axx,B,C Sin Pin

  30. Surface non-magnetic term ・SHG response causes an isotropic contribution only. Bulk non-magnetic term ・　For crystallographic contribution the electric quadrupole should be introduced to get four rank tensor. SHG response causes an anisotropic contribution (parameter B). Surface magnetization induced term ・　The surface magnetic response comes from the electric dipole term expanded by magnetization and contributes to the parameter C.

  31. (a) A=5, B=0, C=0.85 The azimuthal pattern was interpreted in terms of combination of B and C. (b) A=5, B=0.85, C=0.85 Calculated polar patterns of the azimuthal angle-dependence (Sin-Pout) Without nonlocality The equation of the azimuthal angle-dependence by theoretical analysis (a) A=5, B=0, C=0.85 　　・For B much smaller than C, the polar pattern shows 45 rotation for the magnetization reversal. With nonlocality (b) A=5, B=0.85, C=0.85 　　･For B comparable C, the polar pattern undergo a smaller rotation.

  32. 103 SHG intensity (counts/10sec.) (a) Sin-Pout 103 SHG intensity (counts/10sec.) (b) Sin-Sout Azimuthal angle-dependence of MSHG for a [Fe(3.5ML)/Au(3.5ML)] superlattice(Sin-Pout, Sin-Sout configuration) The equation of the azimuthal angle-dependence by theoretical analysis Sin-Pout Sin-Sout ASP(surface nonmagnetic term) =460 ASS(surface nonmagnetic term) =100 B(bulk nonmagnetic term) =26 C(surface magnetic term) =-88

  33. (a) Pin-Pout (b) Pin-Sout 103 103 SHG intensity (counts/10sec.) APP=1310, B=26, C=-88 APS=-300, B=26, C=-88 (c) Sin-Pout (d) Sin-Sout 103 103 SHG intensity (counts/10sec.) ASP=460, B=26, C=-88 ASS=100, B=26, C=-88 Calculated and experimental patterns :x=3.5 Dots：exp. Solid curve：calc.

  34. Nonlinear Kerr rotation (deg.) Nonlinear Kerr rotation (deg.) Nonlinear Kerr ellipticity (c) Exp. (d) Calc. Azimuthal angle (deg.) Calculated and experimental pattern of Nonlinear Kerr rotation and ellipticity (a) Experimental pattern (Sin) The azimuthal angle-dependences of nonlinear Kerr rotation angle and ellipticity in [Fe(3.75ML)Au(3.75ML)] (b) Calculated pattern (Sin)

  35. Nonlinear Kerr rotation (deg.) Nonlinear Kerr rotation (deg.) Fe(2.5ML)/Au(2.5ML) Fe(2. 5ML)/Au(2.5ML) Nonlinear Kerr rotation (deg.) Fe(2.75ML)/Au(2.75ML) Nonlinear Kerr rotation (deg.) Nonlinear Kerr rotation (deg.) Fe(3.25ML)/Au(3.25ML) Fe(3.25ML)/Au(3.25ML) Nonlinear Kerr rotation (deg.) Nonlinear Kerr rotation (deg.) Fe(3.5ML)/Au(3.5ML) Fe(3.5ML)/Au(3.5ML) Experimental and calculated patterns of Kerr rotation angle (b) Calculation (a) Experiment (b) Calculation (a) Experiment Nonlinear Kerr rotation (deg.) Fe(2.75ML)/Au(2.75ML) Sin configuration: (a) Experimental data, (b) Calculated using parametersdetermined by fitting to the azimuth patterns

  36. K(2)=31.1 (a) Exp. Nonlinear Kerr rotation angle (deg.) x=1.75 (b) Calc. Modulated rate x (ML) Nonlinear Kerr rotation angle of [Fe(xML)/Au(xML)] (1.25x3.75) superlattices (Sin) Calculation and experimental result Calculated nonlinear Kerr rotation angle K(2) using the fitting parameter ASP, ASS, B, C of the azimuthal pattern (The maximum K(2) was selected for azimuth angle) ･ The experimental maximum K(2) for x=1.75 superlattice was 31.1. ・ The calculated K(2) reproduced the muximum K(2) for x=1.75 superlattice. The nonlinear Kerr rotation was explained by theoretical analysis. Fig. Nonlinear Kerr rotation angle of [Fe(xML)/Au(xML)] (1.25x3.75) superlattices [(a)Calculation, (b)Experiment]

  37. Summary: MSHG of Fe/Au superlattice The four-fold pattern clearly reflects the symmetry of the MgO(100) substrate. This suggests that the Fe/Au superlattice is perfectly epitactic to the substrate. ・　The azimuthal angle dependence was analyzed in terms of nonlinear electrical susceptibility tensor taking into account the magnetic symmetry of the superlattice. ･The azimuthal pattern was explained by symmetry analysis, taking into account the surface non-magnetic A, bulk non-magnetic B and surface magnetic C contributions.

  38. Summary (cont’d) ・　MSHG was shown to lead to a nonlinear Kerr rotation (2)K that can be orders of magnitude larger than its linear equivalent (0.2), e.g., (2)Kfor x=1.75 was 31.1 • We observed azimuthal angle-dependence of the nonlinear Kerr rotation for the first time. ・The azimuthal angle-dependence of the nonlinear Kerr rotation were explained using parameters determined from azimuthal patterns of MSHG response • Modulation period dependence of parameters: • A (Surface nonmagnetic) is large for short period • B (Bulk nonmagnetic) is nearly constant • C (Surface magnetic) becomes larger with modulation Period.

  39. Fabrication of permalloy nanostructure by Damascene technique ①Preparation of substrate: Spin-coating of ZEP resist with high etching resistance ②EB-exposition: Write patterns by EB ③Development: Formation of mask-pattern by development ④Etching：By dry-etching process mask-pattern is transferred to the substrate ⑤Deposition of magnetic film: Deposition of magnetic films by sputter or evaporation ⑥Polishing: Obtain flat buried structure using chemical-mechanical polishing Process is simplified by abbreviation of lift-off and repeated spin-coating

  40. EB-patterning process Spin coating of resist 〔１〕Dot size 100nm×300nm rectangular dot with 300nm-spacing 100nm square dot with 300nm-spacing 〔２〕Patterned area: 3mm×3mm 〔３〕EB-resist thickness: 300 nm 　　　・・・by spin-coating with 5000 rpm rotation 〔４〕Baking160℃20min EB exposure Development Si substrate

  41. Clean Room Laboratory • Electron beam lithography

  42. Dry etching process Etching Resist removal 300nm 100nm 〔１〕Etching gas: CF4 〔２〕Vacuum3.0×10-3Pa 〔３〕Gas pressure 9.2Pa 〔４〕RF power: 400W 〔５〕Etching rate: 0.1μm/min Silicon surface after etching

  43. Dry-etching

  44. Laboratory EB deposition RF magnetron sputtering

  45. Embedding of permalloy 〔１〕material:permalloy（Ni80Fe20） 〔２〕Vacuum3.0×10-6Torr 〔３〕Accelerating voltage 4kV 〔４〕Deposition rate1.0Å/sec Embedding of permalloy film by electron beam deposition Chemical mechanical polishing 〔１〕Polishing chemicals:Si wafer grain-size～20nm 〔２〕pH11 〔３〕polishing rate:60nm/min flatting

  46. Observation • ＡＦＭ/MFM FE-SEM

  47. 0.6μm 3μm SEM observation 300nm×100nmsquare dot, 300 nm space

  48. 100nm Cross sectional SEM observation Dot depth?

  49. Cross section SEM image of Line and space pattern (width =100nm) ０．３μm

  50. MFM observation of a permalloy film

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